Mecanismes de Fragmentation de l'Oxazole

The fragmentation mechanisms of oxazole have been studied in detail on using various experimental techniques (refocusing of metastable ions, deuteration, AP measurements) and by theoretical calculations.     

Stress in aerogel during depressurization of autoclave: II. Silica gels

Although supercritical drying avoids the capillary stresses that tend to warp and crack xerogels, there are other sources of stress that interfere with the preparation of monolithic aerogels. In this paper, we present experimental results showing that there is a limit to the rate at which the pressure can be released from the autoclave without causing cracking, and that the maximum rate decreases as the gel size increases. Using an analysis developed in a companion paper, the stresses generated during depressurization are compared to the modulus of rupture of our aerogels. The calculations require knowledge of the pressure-dependence of the density of the vapor (ethanol, in our experiments), as well as the permeability and modulus of the gel network. Measurements of those properties were performed on a series of silica gels made under basic and neutral conditions. We find that the calculated stresses are large enough to account for the cracking of our gels at high rates of depressurization; moreover, the predicted dependence of stress on gel diameter is in agreement with experiment.     

Oxidative Stress and Genotoxicity in Melanoma Induction: Impact on Repair Rather Than Formation of DNA Damage?

Keratinocytes and melanocytes, two cutaneous cell types located within the epidermis, are the origin of most skin cancers, namely carcinomas and melanomas. These two types of tumors differ in many ways. First, carcinomas are almost 10 times more frequent than melanomas. In addition, the affected cellular pathways, the mutated genes and the metastatic properties of the tumors are not the same. This review addresses another specificity of melanomas: the role of photo-oxidative stress. UVA efficiently produces reactive oxygen species in melanocytes, which results in more frequent oxidatively generated DNA lesions than in other cell types. The question of the respective contribution of UVB-induced pyrimidine dimers and UVA-mediated oxidatively generated lesions to mutagenesis in melanoma remains open. Recent results based on next-generation sequencing techniques strongly suggest that the mutational signature associated with pyrimidine dimers is overwhelming in melanomas like in skin carcinomas. UVA-induced oxidative stress may yet be indirectly linked to the genotoxic pathways involved in melanoma through its ability to hamper DNA repair activities.     

Structure (x-ray analysis) of Manicoline B,a mixture of two diastereoisomers of a new alkaloid from dulacia guianensis (olacaceae)

Manicoline B, isolated from the root bark of $\text{Dulacia guianensis}$ (Olacaceae), was shown by X-ray analysis to be an equimolecular mixture of two diastereoisomers of a new alkaloid and to have structure (3).     

ALTERATIONS OF PROTEOGLYCANS IN ULTRAVIOLET-IRRADIATED SKIN

Abstract— The effect of UVB exposure on the distribution and synthesis of dermal proteoglycans was measured in the skin of hairless mice. Two groups of mice were included: one was irradiated for 10 weeks; the other was kept as control. After intraperitoneal injection of sodium ³⁵S-sulfate, punch biopsies were taken for histology and proteoglycans were extracted from the remaining skin with 4 M guanidinium chloride, containing 3–[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (0.5%, weight per volume). Following proteolytic digestion, the glycosaminoglycan constituents were isolated and analyzed by quantitative cellulose acetate electrophoresis and enzymatic digestibility.
Under the influence of UVB radiation, newly synthesized proteoglycans measured by ³⁵SO₄ uptake increased as much as 60%. In addition, the irradiated skin had a higher average content of proteoglycan than had control skin (4981 μg vs 4134 μg/g dry weight). This could be ascribed to an increase in heparin (1400 vs 533 μ g/g dry weight) and heparan sulfate (472 vs 367 μg/g dry weight), whereas no change in the concentration of hyaluronic acid (1243 vs 1372 μg/g dry weight) and dermatan sulfate (1866 vs 1863 μg/g dry weight) was observed. The irradiated animals also exhibited a marked increase in the synthesis of heparan sulfate and heparin (62% and 71%, respectively). These results demonstrate that chronic doses of UVB altered proteoglycan metabolism through both quantitative and qualitative changes.     

1,3-Dipolar Cycloadditions VI [1].Structure and Conformation of Cycloadducts from Reactions of C-Aryl-N-phenylnitrones with Substituted Cinnamic Acid Amides

Summary.  Studies on cycloadditions of C,N-diarylnitrones to cinnamic acid amides were carried out. The diastereoisomeric (I, II) and (in some cases) regioisomeric (III) cycloadducts obtained were characterized by spectroscopic and X-ray data. Conformational studies were carried out by molecular modelling. Received February 8, 2000. Accepted February 18, 2000     

Koordinative Verankerung von Nickelkomplexen an Hectorit-Schichtsilicaten mit Hilfe langkettiger Diphenylphosphinogruppen

Coordinative Anchoring of Nickel Complexes on Hectorite-Layer Silicates through long-chain Diphenylphosphine Groups The treatment of sodium and calcium hectorites with 2-diphenylphosphinoethyltriethoxysilane leads, by condensation of free hydroxyl groups, to functionalized layer silicates with long-chain diphenylphosphine groups onto which nickel chloride can be coordinated. In these modified hectorites the fourth coordination site of the tetrahedrally coordinated nickel is occupied by a labile solvent molecule. On the other hand, the complex bis(2-diphenylphosphinoethyltriethoxysilane)nickel(II) chloride prepared beforehand also reacts with sodium hectorite under condensation; but the square-planar coordination of the four stable ligands (two chloride and two phosphor atoms) is maintained. This finds its expression in a significant activity decrease in catalytic reactions as compared to the aforementioned products.     

β-Thiopeptides: Synthesis,NMR Solution Structure,CD Spectra,and Photochemistry

To test the effect of NH−C=S groups (Scheme 1) on the stability of β-peptide secondary structures, we have synthesized three β-thiohexapeptide analogues of H-(β-HVal-β-HAla-β-HLeu)₂-OH ( 1 ) with one, two, and three C=S groups in the N-terminal positions (cf. 2 – 4 and model in Fig. 1). The first C=S group was introduced selectively by treatment with Lawesson reagent of Boc-β-dipeptide esters ( 6 and 8 ). A series of fragment-coupling steps (with reagents as for the corresponding sulfur-free building blocks) and another thionation reaction led to the title compounds with a C=S group in residues 1, 1, and 3, as well as 1, 2, and 3 of the β-hexapeptide (Schemes 2 and 3). The sulfur derivatives, especially those with three C=S groups, were much more soluble in organic media than the sulfur-free analogues (>1000-fold in CHCl₃; Table 1). The UV and CD spectra (in CHCl₃, MeOH, and H₂O) of the new compounds were recorded and compared with those of the parent β-hexapeptide 1 (Figs. 2 – 4); they indicate the presence of more than one secondary structure under the various conditions. Most striking is a pronounced exciton splitting (Δλ ca. 20 nm, amplitude up to +121000) of the ππ*_C=S band near 270 nm with the β-trithiohexapeptide (with and without terminal protecting groups), and strong, so-called `primary solvent effects', in the CD spectra. The CD spectrum of the β-dithiohexapeptide 3 undergoes drastic changes upon irradiation with 266-nm laser light of a MeOH solution (Fig. 5). The NMR structure in CD₃OH of the unprotected β-trithiohexapeptide 4 was determined to be an (M)-3₁₄-helix (Fig. 7), very similar to that of the non-thionated analogue (cf. 1 ). NMR and mass spectra of the β-hexapeptides with C=S and with C=O groups are compared (Figs. 6 and 8).